1. Field of the Invention
The present invention relates to a vehicle pole antenna, and more particularly to a vehicle pole antenna having a helical antenna element.
2. Description of the Related Art
A vehicle pole antenna is limited in terms of physical length, so that a helical antenna element capable of reducing an antenna's physical length as compared to an antenna effective length is generally used for the vehicle pole antenna. Further, the vehicle pole antenna is generally designed to have flexibility in consideration of a possibility that the antenna itself collides with an object. However, if the vehicle pole antenna having the helical antenna element bends due to collision, a winding pitch of the helical antenna element may be shifted to change the antenna effective length thereof, which may result in a change of electrical characteristics.
To solve the above problem, Patent Document 1 discloses a helical antenna in which a coated wire obtained by coating a core wire with an insulating coating material is helically and closely wound and in which a winding pitch thereof is determined by the thickness of the coating material. With this configuration, the winding pitch can be kept constant even when the antenna bends.
Patent Document 2 discloses a vehicle pole antenna including an antenna element which has a helical antenna element constructed by inserting a helical coil with elasticity into a helical groove of a rod and in which a coil spring is provided between the rod and a base side element metal fitting. With this configuration, the antenna element can bend at the coil spring part to prevent the winding pitch thereof from being changed to reduce a change of electrical characteristics and to remove bending tendency.